Current devices are able to provide good estimates of "absolute" pressure and good measures of differential pressure, but not at the same place. Currently, two separate pressure sensors are used in these situations. In situations where two sensors could be employed to monitor the pressure of a process, a single absolute pressure sensor combined with a differential pressure sensor at a nearby but different location may yield satisfactory results, but at a substantially higher cost than two sensors at one location. Every fixture associated with a high pressure process must be carefully designed into the process so as not to either disrupt it or cause a leak or loss of pressure. Economic justification for a chip containing both an absolute and differential pressure sensor is therefore quite easy to see where process pressures are about 3,000 psi or greater, where the differential pressure needing to be measured is on the order of 10-15 psi. Of course, the invention herein has wider applicability than simply to measure pressures on the order of 10-15 psi in an absolute pressure range of approximately 3,000 psi, however, the economic justification for use of the invention is well founded in those ranges.
Using polycrystalline silicon long rectangular diaphragms, it has been found that lengthwise piezoresistive elements mounted thereon can produce repeatedly accurate indications of pressures in large (or overpressure) pressure ranges. For example, using a process as described in U.S. Pat. No. 4,592,238 or something similar, a pair of references (Guckel and Burns, DOCTORAL THESIS Micromechanics of Integrated Sensors and the Planar Processed Pressure Transducer, May 1988, and HIGH-STRESS AND OVERRANGE BEHAVIOR 0F SEALED-CAVITY POLYSILICON PRESSURE SENSORS, Chau, et al, IEEE Solid-state Sensor And Actuator Workshop, Jun. 4, 1990) have shown that aspect ratios of 3 to 1 would provide sufficient length for piezoresistive strip elements to generate repeatably measurable signals of appropriate size for such very high pressure conditions. As used herein, the term "aspect radio" refers to length/width ratio of a given diaphragm plate. These papers did not produce long rectangular diaphragm plates with aspect ratios of higher than 4 to 1, such as greater than 10 to 1, although there is speculation that such could be produced.
Because of the way that the piezoresistive sensor strips would be located on the long rectangular plates, the widthwise strain that is induced produces the variation in signal indicative of the amount of pressure being applied to the diaphragm. In the overpressure range, the diaphragm used for sensing differential pressures in the small (differential) range will be substantially flattened out against the overpressure protection stop of the base of the cavity over which the plate or diaphragm is located. It is important that the static (or absolute) sensor response be linear well into the overpressure range of the differential signal. It is useful to have an integral overpressure mechanical stop on the differential sensor to prevent breakage of the differential sensing diaphragm at high pressure. At the same time, under high pressure, the static device should continue to give correct readings. In this way, the differential sensor can give a "high" signal when the differential pressure is, for example, .ltoreq.15 psi, and also withstand the condition where the differential pressure is 3,000 psi. It is also important to note that the existence of two sensors (a high pressure static sensor and a low pressure differential sensor) helps in two ways: 1) providing compensation signals for package tube effects, and 2) providing high signal static output for multi-variable sensing.
In the "Guckel" method, polysilicon is low pressure chemical vapor deposited (LPCVD) onto either the post oxide directly, or some intermediate, possibly insulating, layer which overlays the post oxide. The available signal level from piezoresistors on polysilicon is typically a factor of three times less than what is available from single crystal silicon of proper orientation. The polysilicon may then be annealed with a laser to produce what may be a single crystalline structure. Its orientation may not be correct which would cause problems with measuring strain. Alternative methods of melting or annealing the polysilicon could also be used which may or may not alleviate the crystal orientation problem.
In the method described in this patent, wafer bonding is used instead. Therefore, for nearly any aspect ratio, no backfill material or post oxide is required for the construction of the long rectangular diaphragm. The diaphragm is merely laid over the cavity as will be described later.